We investigated the role of anonymity, reciprocity, and conformity for voluntary contributions,based on a natural field experiment conducted at a national park in Costa Rica. Contributions made in public in front of the solicitor were 25 percent higher than contributions made in private. Giving subjects a small gift before requesting a contribution increased the likelihood of a positive contribution. At the same time, however, the conditional contribution decreased. The total effect of giving a gift was positive but small, and taking the cost of the gift into account, it was far from profitable. When the subjects were told that the typical contribution of others was $2 (a small contribution), the probability of a contribution increased and the conditional contribution decreased, compared to providing no reference information. Providing a high reference level ($10) increased the conditional contributions. Overall, the total effects have the expected signs, although the magnitudes are smaller than what one might have expected based on existing evidence from laboratory experiments.